docs: def load_trajectory(name, skip=1, format=None):
      '''Load a trajectory file into chemlab. You should call this
      command after you load a `~chemlab.core.System` through
      load_system or load_remote_system.
  
      '''
      df = datafile(name, format=format)
      dt, coords = df.read('trajectory', skip=skip)
      boxes = df.read('boxes')
      viewer.current_traj = coords
      viewer.frame_times = dt
      
      viewer.traj_controls.set_ticks(len(dt))
      
      def update(index):
          
          f = coords[index]        
          
          for fp in _frame_processors:
              f = fp(coords, index)
  
          # update the current representation
          viewer.representation.update_positions(f)
          viewer.representation.update_box(boxes[index])
          current_system().r_array = f
          current_system().box_vectors = boxes[index]
          viewer.traj_controls.set_time(dt[index])
          viewer.update()
      
      viewer.traj_controls.show()
      viewer.traj_controls.frame_changed.connect(update)